Medical devices and methods of making the same

ABSTRACT

A method of making a medical device includes compressing a prosthesis, e.g., a therapeutic agent-carrying stent or stent-graft, to a catheter without contacting an outer surface of the prosthesis, e.g., by electromagnetically compressing the prosthesis.

TECHNICAL FIELD

[0001] This invention relates to medical devices, such asendoprostheses, and methods of making the same.

BACKGROUND

[0002] The body includes various passageways such as arteries, otherblood vessels, and other body lumens. These passageways sometimes becomeoccluded or weakened. For example, the passageways can be occluded by atumor, restricted by plaque, or weakened by an aneurysm. When thisoccurs, the passageway can be reopened or reinforced, or even replaced,with a medical endoprosthesis. An endoprosthesis is typically a tubularmember that is placed in a lumen in the body. Examples of endoprosthesisinclude stents and covered stents, sometimes called “stent-grafts”.

[0003] Endoprostheses can be delivered inside the body by a catheterthat supports an endoprosthesis in a compacted or reduced-size form asthe endoprosthesis is transported to a desired site. Upon reaching thesite, the endoprosthesis is expanded, for example, so that it cancontact the walls of the lumen.

[0004] The expansion mechanism may include forcing the endoprosthesis toexpand radially. For example, the expansion mechanism can include thecatheter carrying a balloon, which carries a balloon expandableendoprosthesis. The balloon can be inflated to deform and to fix theexpanded endoprosthesis at a predetermined position in contact with thelumen wall. The balloon can then be deflated, and the catheterwithdrawn.

[0005] In another technique, a self-expandable endoprosthesis is formedof an elastic material that can be reversibly compacted and expanded.During introduction into the body, the endoprosthesis is restrained in acompacted condition on a catheter. Upon reaching the desiredimplantation site, the restraint is removed, for example, by retractinga restraining device such as an outer sheath, enabling theendoprosthesis to self-expand by its own internal elastic restoringforce.

[0006] During delivery, an endoprosthesis is typically attached to acatheter to prevent the endoprosthesis from slipping off or shifting onthe catheter, which can cause loss of the endoprosthesis, and/orinaccurate and imprecise delivery of the prosthesis. Attachment of theendoprosthesis can include mechanically clamping or crimping theendoprosthesis on the catheter.

SUMMARY

[0007] This invention relates to medical devices, such asendoprostheses, and methods of making the same.

[0008] In one aspect, the invention features a method of making amedical device including securing an endoprosthesis to a support, suchas a catheter, without mechanically contacting an outer surface of theendoprosthesis. Securing the endoprosthesis may include compressing theendoprosthesis. Examples of other supports include balloons, guidewires,and sheath introducers.

[0009] In another aspect, the invention features a method of making amedical device including electromagnetically compressing a prosthesis toa catheter.

[0010] In another aspect, the invention features a method of making amedical device including positioning a prosthesis including atherapeutic agent on an expandable portion of a catheter, andelectromagnetically compressing the prosthesis.

[0011] Embodiments of the aspects of the invention may include one ormore of the following features. The method includes electromagneticallycompressing the prosthesis on an inflatable balloon on the catheter. Themethod includes heating the expandable portion or the balloon. Themethod includes positioning a restraining device over the compressedprosthesis. The method includes compressing, e.g., electromagnetically,different portions of the prosthesis with different forces. The methodfurther includes supporting the prosthesis on the catheter. The methodfurther includes axially and/or radially translating the prosthesis. Themethod further includes positioning a mandrel in the catheter.

[0012] The prosthesis can be a balloon-expandable stent, aballoon-expandable stent-graft, a self-expandable stent, aself-expandable stent-graft, or combinations thereof. The prosthesis caninclude a therapeutic agent.

[0013] In another aspect, the invention features medical devices madeaccording to the methods described herein.

[0014] In another aspect, the invention features a method of making amedical device including securing a medical balloon to a catheter, andsubsequently securing a marker band under the balloon to the catheter.

[0015] Embodiments may include one or more of the following features.The band is secured to the catheter without mechanically contacting theband. The band is secured to the catheter electromagnetically. The bandis visible by magnetic resonance imaging. The band is formed of amaterial having dysprosium or gadolinium. The band is formed of amaterial selected from the group consisting of gold, platinum, tungsten,and tantalum.

[0016] In another aspect, the invention features a method of making amedical device including securing a marker band to a support withoutcontacting an outer surface of the band. The support can be, e.g., acatheter, a medical balloon, a guidewire, or a stent.

[0017] In another aspect, the invention features a method includingproviding an endoprosthesis including a metal body and a polymer layer,and reducing the size of the endoprosthesis without contacting theendoprosthesis. The polymer layer may include a drug. The polymer layermay be on an outside surface of the metal body. The endoprosthesis, forexample, a stent, can be reduced in size without contacting the polymerlayer. The method can further include securing the endoprosthesis to asupport, e.g., a catheter or a medical balloon.

[0018] Embodiments may have one or more of the following advantages. Anendoprosthesis can be attached, e.g., crimped, to a catheter withoutcontacting an exterior of the endoprosthesis. Not contacting theexterior of the prosthesis can reduce damage to the prosthesis, forexample, due to relatively high shearing forces normal to and/ortangential to the surface of the prosthesis. Reducing shearing forcescan be particularly advantageous for prostheses carrying a drug, such asdrug-coated stents and stent-grafts in which the drug, e.g., ashear-sensitive material, such as DNA or RNA, is applied to a surface oris embedded in a matrix, e.g., polymer matrix, applied to theprosthesis. Not contacting the exterior of the prosthesis can alsoreduce cross contamination between prostheses, and/or between aprosthesis and a conventional crimper.

[0019] As described herein, in embodiments in which the prosthesis isself-expandable, the methods can allow portions of the prosthesisalready loaded in a restraining device, e.g., a sheath, to becompressed. This may allow unloaded portions of the prosthesis to beloaded into the restraining device without the inner wall of therestraining device rubbing against the exterior surface of the loadedportions of the prosthesis. As a result, damage to the surface of theprosthesis is reduced.

[0020] The methods can be relatively quick and reproducible, with goodaccuracy and precision. For example, compression forces, which can beproportional to the square of a discharge current, can be accurately andprecisely controlled by controlling the discharge current. In someembodiments, the prosthesis can be quickly and permanently deformedwithout the prosthesis substantially springing back. The methods can beused to compress or to crimp prostheses with various diameters.

[0021] Other features and advantages of the invention will be apparentfrom the description of the preferred embodiments thereof and from theclaims.

DESCRIPTION OF DRAWINGS

[0022]FIG. 1 is a partially cut-away, perspective view of an embodimentof a system for making a medical device.

[0023]FIG. 2 is a cross-sectional view of the system of FIG. 1, takenalong line 2-2.

[0024]FIG. 3 is a cross-sectional, schematic view of an embodiment of asystem for making a medical device.

[0025]FIG. 4 is a cross-sectional, schematic view of an embodiment of asystem for making a medical device.

[0026]FIGS. 5A, 5B, and 5C are schematic views of an embodiment of amethod for making a medical device.

[0027]FIG. 6 is a cross-sectional, schematic view of an embodiment of asystem for making a medical device.

[0028]FIG. 7 is a cross-sectional, schematic view of an embodiment of asystem for making a medical device.

[0029]FIG. 8 is a cross-sectional, schematic view of an embodiment of asystem for making a medical device.

[0030]FIG. 9 is a schematic diagram of an embodiment of a system formaking a medical device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031]FIGS. 1 and 2 show a system 20 for making a medical device 22. Inthe embodiment illustrated, a prosthesis 28, such as a stent or astent-graft, is secured, e.g., crimped, to a balloon 30 of a catheter 32using electromagnetic forces. During securement of prosthesis 28 tocatheter 32, the outer surfaces of the prosthesis experience no physicalcontact or shear forces. As a result, contamination of or damage tomedical device 22, particularly to prosthesis 28, is minimized.

[0032] System 20 includes a tube 24 formed of wound wire 26. The ends ofwire 26 are connected to a current source 27. Current source 27 can be apower supply of constant or variable current controlled by a controller29, which may include a capacitor bank and appropriate switches.

[0033] Referring particularly to FIG. 1, prosthesis 28, is uncompressed,e.g., as formed, or partially compressed, and is positioned over balloon30. For example, prosthesis 28 can be centered on balloon 30. Balloon 30is not fully inflated, i.e., the balloon can be partially or completelydeflated. To secure prosthesis 28 to balloon 30 and catheter 32, acurrent is discharged through wire 26 of tube 24. The discharged currentgenerates a rapidly changing magnetic flux that penetrates electricallyconductive material in prosthesis 28, e.g., the stainless steel of astent. Consequently, an eddy current is induced in prosthesis 28. Thisinduced current has an associated induced magnetic field that has apolarity that opposes the polarity of the magnetic field of tube 24.Referring as well to FIG. 2, as a result, magnetic repulsion between(stationary) tube 24 and prosthesis 28 forces the prosthesis radiallyinward to attach, e.g., by crimping, the prosthesis to the balloon 30and catheter 32.

[0034] Repulsive magnetic forces are generated between a forming coil(e.g., tube 24) and a work piece positioned inside the forming coil(e.g., prosthesis 28) to perform mechanical work, according to “Lenz'sLaw of Repulsion”. In operation, a rapidly changing, unidirectionalcurrent is applied to the forming coil to generate a rapidly changingmagnetic flux. The relatively high rate of change of flux can beproduced, for example, by rapidly discharging a relatively largeelectric charge from an energy storage capacitor through a lowresistance coil. As a result, this large current causes a rapidlychanging magnetic flux (e.g., arrow B, FIGS. 1 and 2). When a rapidlychanging magnetic flux penetrates an electrically conductive material, asimilarly changing current, e.g., eddy current, is induced in thematerial (e.g., arrow I_(in), FIGS. 1 and 2). The induced current can beproportional to the initial intensity and time rate of change of themagnetic flux. That is, the higher the rate of change, the greater theinduced current.

[0035] The induced current has an associated induced magnetic field(e.g., arrow B_(in), FIGS. 1 and 2). According to Lenz's Law, theinduced magnetic field must oppose the change in flux from the formingcoil. Here, the induced magnetic field (B_(in)) has a polarity oppositethe polarity of the magnetic field (B) inducing the current. Also, themagnetic flux from the induced current is directed radially outward fromthe work piece against the magnetic flux of the forming coil. As aresult, magnetic repulsion between the work piece and the forming coilcan force the work piece radially inward, i.e., compress the work piece.

[0036] The force generated between the forming coil and the work piececan be controlled by controlling the applied current and/or the distancebetween the coil and the work piece. For example, in some embodiments,the force is proportional to the square of the discharge current, i.e.,the higher the discharge current, the greater the magnetic force. Theforce can also be inversely proportional to a separation distancebetween the forming coil and the work piece, i.e., the closer the workpiece is from the forming coil, the more force the work piece canexperience.

[0037] The current-to-compression relationship and/or the separationdistance-to-compression relationship for controllably compressing a workpiece can be determined empirically, such as by observing the degree ofdeformation in the work piece as a function of applied current and/ordistance between the work piece and forming coil. For example, a pulseof current can be applied, and the change, e.g., decrease, in thediameter of the work piece can be measured as a function of the appliedcurrent and/or the distance between the work piece and forming coil. Thedegree to which the work piece displaces can be measured, for example,by using an optical fiber pass through the forming coil. Duringfabrication, this data can be used to apply a predetermined currentbased on the size of the work piece, e.g., the separation distance, andthe degree of compression desired. As a result, using magnetic forces todeform a work piece can provide reproducibly accurate results. Thedegree of compression can also be a function of the endoprosthesismaterial, e.g., stainless steel, which can also be determinedempirically.

[0038] Compression of the work piece can be performed in one step or asa series of steps. For example, using predetermined data, the work piececan be compressed from a starting dimension to a final dimension in onestep by applying a sufficient large current. Alternatively, the workpiece can be sequentially compressed by applying a series of currentpulses, each sufficient to compress the work piece until the desiredsize is achieved. Each current pulse can be adjusted to provide thedesired compression using the predetermined data relating to compressionas a function of current, separation distance, and/or endoprosthesismaterial.

[0039] The use of magnetic forces to deform a work piece is a techniqueknown as “magnetic pulse forming”, which is described, for example, inBatygin Yu et al., “The Experimental Investigations of the MagneticPulse Method Possibilities for Thin-walled Metal Plates Deformation”,Technical Electro-dynamics, 1990, #5, p. 15-19, hereby incorporated byreference.

[0040] Medical device 22 includes prosthesis 28 and inflatable balloon30 attached to catheter 32. Prosthesis 28, balloon 30 and catheter 32can be conventional. For example, prosthesis 28 can be a conventionalstent or a conventional stent-graft. The stent can be made of anelectrically conducting material such as Nitinol or Elgiloy™ stainlesssteel. The stent-graft can be a stent attached to a biocompatible,non-porous or semi-porous polymer matrix made of polytetrafluoroethylene(PTFE), expanded PTFE, polyethylene, urethane, or polypropylene.Prosthesis 28 can be balloon expandable, self-expandable, or acombination of both. Examples of prosthesis 28 are described in U.S.Pat. Nos. 5,725,570 and 5,234,457, all hereby incorporated by reference.Prosthesis 28 can include a releasable therapeutic agent or apharmaceutically active compound, such as described in U.S. Pat. No.5,674,242, and commonly-assigned U.S. Ser. No. 09/895,415, filed Jul. 2,2001, all hereby incorporated by reference. The therapeutic agents orpharmaceutically active compounds can include, for example,anti-thrombogenic agents, antioxidants, anti-inflammatory agents,anesthetic agents, anti-coagulants, and antibiotics. Device 22 can alsoinclude side ports adapted for perfusion.

[0041] In some embodiments, catheter 32 can be fitted with a mandrel,e.g., a hypotube, placed inside the catheter. The mandrel, which can actas a die about which the prosthesis is deformed, can limit the degree towhich the prosthesis is compressed, e.g., so that the prosthesis doesnot become overly compressed. The mandrel can include wire coiled aboutits axial axis to generate repulsive magnetic forces from within thecatheter or the prosthesis. The magnetic forces can be used to force theprosthesis radially outward.

[0042] Referring to FIG. 3, in some embodiments, prosthesis 28 can bepositioned over balloon 30 using a temporary sheath 34 prior tocompression. Sheath 34 can be used as a centering device. Sheath 34, forexample, an electrically non-conducting tube such as a polymer or glasstube, has an inner diameter larger than the outer diameter of prosthesis28. For example, the sheath can have a diameter sufficiently large toload the prosthesis into the sheath without subjecting the prosthesis todamaging shear forces. After prosthesis 28 is loaded in sheath 34, thesheath is positioned such that the prosthesis is positioned over balloon30, which can be partially or fully compressed. Prosthesis 28 can becompressed and secured to balloon 30 as described above, and sheath 34can then be withdrawn from tube 24. In embodiments, sheath 34 contactsonly selected portions of prosthesis 28, e.g., the proximal and/ordistal end of the prosthesis. In embodiments, sheath 34 can serve as afield shaper. Sheath 34 can be made of a material and/or has aconfiguration that enhances the electromagnetic field.

[0043] Magnetic pulse forming can also be used to secure self-expandingprostheses to a catheter. Referring to FIG. 4, a self-expandableprosthesis 40 is positioned over a predetermined portion of a catheter42, e.g., near the distal end of the catheter. Prosthesis 40 andcatheter 42 can be conventional devices used with self-expanding stentsand stent-grafts. In some embodiments, catheter 42 can be fitted with amandrel, as described above.

[0044] During fabrication, prosthesis 40 is magnetically compressed tocatheter 42, as described above. When prosthesis 40 is compressed to apredetermined size, a restraining device 44, e.g., an outer sheath, ismoved over the prosthesis (arrow X) to hold the prosthesis in acompressed state. In some embodiments, the applied current issufficiently varied to provide a changing magnetic flux to keep theprosthesis sufficiently compressed so that the restraining device can bemoved into place over the prosthesis.

[0045] In embodiments, a prosthesis can be compressed sequentially alongits axial axis, and a sheath can be sequentially positioned overcompressed portions of the prosthesis. Referring to FIGS. 5A-5C, aprosthesis 100 and a restraining device 102, e.g., a polymer sheath, arepositioned over a catheter 104 (FIG. 5A). A portion of prosthesis 100,e.g., the proximal end, can then be compressed. For example, prosthesis100, device 102, and catheter 104 can be introduced incrementally intotube 106 and moved proximally (arrow Y). As portions of prosthesis 100experience a magnetic flux from tube 106 and compress, restrainingdevice 102 can be slid over the compressed portions (FIG. 5B). Thisprocess of compressing increasingly longer portions prosthesis 100 andsliding restraining device 104 over the compressed portions continuesuntil the restraining device covers prosthesis 100 (FIG. 5C). Thisprocess takes advantage of the property of a magnetic flux to penetrateelectrically non-conducting materials, e.g., a polymer restrainingdevice, and to selectively interact with electrically conductingmaterials. As a result, an electrically conducting prosthesis can beloaded into an electrically non-conducting restraining device withoutexposing the prosthesis to damaging shear forces, even when portions ofthe prosthesis is inside the restraining device.

[0046] Other methods of selectively compressing different portions ofprosthesis 100 include modifying tube 106 to include differentdiameters, coil densities, or independently-operable coil sections, asdescribed below.

[0047] Referring to FIG. 6, in certain embodiments, self-expandingprosthesis 40 can be compressed on catheter 42 using a temporary sheath46 prior to compression. Sheath 46 is generally the same as sheath 34described above and can be used in a similar manner.

Other Embodiments

[0048] In certain embodiments, the forming coil, e.g., tube 24, can bemodified to provide enhanced control of the magnetic fluxes and, as aresult, the forces that are generated. Modifying the forming coil can beuseful where it is desirable to apply non-uniform forces along the axialaxis. For example, some prostheses may be formed of different materials,and the materials may require different forces for compression. Someprostheses may be formed with portions of different structures, e.g., acombination of self-expandable and balloon-expandable portions, and theportions may require different forces for compression. Examples aredescribed in Andersen, U.S. Pat. No. 5,366,504, hereby incorporated byreference.

[0049] In some embodiments, the forming coil can be modified to have anon-uniform inner diameter. That is, the forming coil can have portionswith different diameters. The portions with different diameters can varystep-wise (FIG. 7). Alternatively or in addition, portions withdifferent diameters can vary as a taper, relatively gradually orsuddenly. Since the force can be inversely proportional to the distancebetween the forming coil and the work piece, portions of the work piececloser to the forming coil experience more force than portionsrelatively farther away from the forming coil, for otherwisesubstantially identical conditions. Catheter 52 and prosthesis 54 can beconventional, such as those used in balloon-expandable orself-expandable devices. Portions of the prosthesis requiring greatercompression forces are positioned relatively closer to the forming coil.

[0050] Alternatively or in addition, the forming coil can be modified tohave a varying number of wire coils per unit distance. Generally, forotherwise substantially identical conditions, the higher the number ofwire coils per unit distance, the greater the magnetic force generatedper given pulse of current. For example, referring to FIG. 8, portions62 of prosthesis 54 requiring relatively high compression forces can bealigned with portions 64 of tube 60 having a relatively high number ofwire coils per unit distance. Portions 66 of prosthesis 54 requiringrelatively low compression forces can be aligned with portions 68 oftube 60 having a relatively low number of wire coils per unit distance.

[0051] Alternatively or in addition, the forming coil can be formed,along its axial length, of independently operable sections of wire coilsconnected to independently controllable current sources. The amount ofmagnetic flux and compression forces generated can be controlled byapplying predetermined current pulses to predetermined sections of wirecoils. Relatively high current pulses are applied to sections whererelatively high compression forces are wanted.

[0052] In some embodiments, the current induced in the work piece, e.g.,the prosthesis, can increase the temperature of the work piece. Thisheat can affect securement of the prosthesis to the balloon, forexample, by softening the balloon such that the bond between theprosthesis and the balloon can be enhanced. Alternatively or inaddition, the balloon and/or the catheter can be heated, e.g., usinginfrared radiation or a heatable mandrel. In embodiments, low currentpulses can be applied to the forming coil. For example, low currentpulses can be applied, e.g., alternating with high current pulses, toheat the endoprosthesis to further embed the endoprosthesis into theballoon.

[0053] The forming coil may have non-circular cross sections, e.g.,elliptical, oval, or polygonal having three, four, five, six, or moresides that can be equal or unequal.

[0054] The medical devices described above can include radiopaquemarkers to help the user position the devices. For example, the wire ofthe stent or stent-graft can be radiopaque, e.g., by including gold ortantalum.

[0055] Magnetic pulse forming can also be used to attach band(s) (e.g.,marker bands that are radiopaque or magnetopaque, i.e., visible bymagnetic resonance imaging (MRI)) to various supports. Examples ofsupports include catheters, balloons, guidewires, sheath introducers,temporary filters (e.g., non-metallic, such as ceramic or polymeric,filters), stents, and grafts. In some embodiments, the band(s) can beplaced on the support, e.g., slipped-fit around a polymer shaft, and theband(s) can be attached in a forming coil as described herein. Suitablematerials for the bands include, for example, gold, platinum, tungsten,tantalum, and metal alloys containing a sufficient percentage of heavyelements. Suitable magnetopaque materials include, for example,non-ferrous metal-alloys containing paramagnetic elements (e.g.,dysprosium or gadolinium) such as terbium-dysprosium, dysprosium, andgadolinium; non-ferrous metallic bands coated with an oxide or a carbidelayer of dysprosium or gadolinium (e.g., Dy₂O₃ or Gd₂O₃); non-ferrousmetals (e.g., copper, silver, platinum, or gold) coated with a layer ofsuperparamagnetic material, such as nanocrystalline Fe₃O₄, CoFe₂O₄,MnFe₂O₄, or MgFe₂O₄; and nanocrystalline particles of the transitionmetal oxides (e.g., oxides of Fe, Co, Ni).

[0056] Magnetic pulse forming can be particularly useful for attachingmarker bands to an inner shaft of a balloon catheter (i.e., underneath aballoon), after the balloon has been attached (e.g., welded) to theinner shaft and an outer shaft. Marker bands are typically attached tothe inner shaft, e.g., at an axial position corresponding to thetransition between the balloon and the balloon cone, by a swagingprocess before the balloon is attached to the catheter. With thismethod, however, it can be difficult to adjust the positions of themarker bands after they are attached, e.g., to compensate for balloonsof different lengths. Using non-contact crimping, the marker bands canbe secured in place relative to the balloon after the balloon has beenattached.

[0057] Numerous methods can be used. For example, to move the markerbands axially along the inner shaft underneath the balloon, (a) inflatethe balloon at low pressure, if necessary; (b) determine the position ofthe marker band(s) from the desired final position(s); and (c) grip themarker band(s) with soft mini-grippers (the balloon can be squeezed atlow pressures) and incrementally slide the band(s) along the inner shaftto the final position; (d) position the balloon with the marker band(s)in place within the magnetic pulsing system, inflate the balloon tocenter the bands in the middle of the coil, and non-contact crimp theband(s) to the inner shaft; and (e) repeat any step(s) as necessary. Inother embodiments, the band(s) can be moved along the inner shaft byinserting a polymer wire, or another manipulating device, through alumen of the catheter, and pushing the marker band(s) (preferablypositioned proximally) in position. In other embodiments, the band(s)can be moved along the inner shaft by positioning the balloon such thatthe marker band(s) is outside the forming coil, and by pulsing the coil.The magnetic field from the coil can axially displace (e.g., attract orrepel) the marker band(s). The marker band(s) can also be axiallydisplaced by using permanent magnets and/or electrostatic forces.

[0058] Other electrically conducting medical devices can also be formedby magnetic pulse forming. For example, electrically conductingguidewires having non-linear portions, e.g., curved tips, can be formedby selectively deforming the axial length of a guidewire about a mandrelor a mold having a predetermined profile. In embodiments, the formingcoil can be modified to include selected diameters, coil densities,and/or independently operable coil sections, as described herein.

[0059] Magnetic pulse can also be used to deform electricallynon-conductive devices, e.g., a polymer tube or a PTFE stent orstent-graft. For example, to attach a polymer tube to a catheter, thecatheter can be placed inside the polymer tube. The catheter and thepolymer tube can then be placed inside an electrically conductivemember, such as a metal tube or an electrically conductingself-expanding stent. Using magnetic pulse forming, the electricallyconductive member can be deformed to deform, e.g., compress, the polymertube to the catheter. In embodiments where the electrically conductivemember is a self-expandable member, the member can then expand by itsinternal elastic forces and be removed. The electrically conductivemember and/or the non-conductive device can be coated, e.g., with PTFE,to prevent sticking between the member and the device. Alternatively, orin addition, a forming coil can be placed inside the catheter to forcethe electrically conductive member radially outward using magnetic pulseforming. The forming coil can also serve as a mandrel to limit thedegree of deformation, as described above.

[0060] Magnetic pulse forming can be used with conventional crimpingmethods. For example, a prosthesis can be partially compressed usingmechanical crimpers, positioned over a catheter, and then furthercompressed using magnetic pulse forming.

[0061] During operation, the prosthesis can be moved within the formingcoil, e.g., vibrated axially and/or radially, to provide varying eddycurrents and magnetic forces, which can enhance, e.g., make moreuniform, compression. Alternatively or in addition, the applied currentcan be controlled, e.g., variably, to provide different radialcompression forces.

[0062] In embodiments, a coil of wire can be placed in a catheter toexpand an endoprosthesis, e.g., in a body lumen.

[0063] The following example is illustrative and not intended to belimiting.

EXAMPLE

[0064] Referring to FIG. 9, a stent 100 held by a glass tube 106 iscentered over a balloon 102 of a catheter 104. Stent 100, tube 106, andcatheter 104 are positioned inside a forming coil 108. Balloon 102 andcatheter 104 can be centered in coil 108 using a ceramic mandrel (notshown). Stent 100, e.g., an Express stent available from BostonScientific Corp., has a length of 8 mm and a starting (uncompressed)radius, R₁, of 2 mm, and coil 108 has a radius, R₂, of 2.1 mm.

[0065] Considering the actual outer surface and open areas of stent 100,it is believed that a final pressure of 34 N/mm² is used to crimp thestent to a final radius of 0.5 mm. This pressure is used in the last 0.1mm reduction in radius, when stent 100 contacts balloon 102. Beforestent 100 contacts balloon 102, the force used to deform the stent froma radius of 2 mm to 0.6 mm is believed to be less than 3 N. The surfacearea of stent 100 (8.79 mm²) during compression remains relatively thesame since the open areas of the stent decrease during compression.Accordingly, the amount of energy required to crimp ispAd=[(0.2·34)+(1.4·3)]×8.79=97 N·mm=0.097 Joule.

[0066] As described herein, when a current is generated in forming coil108, a current is induced in stent 100, which generates a magnetic fieldopposing the magnetic field of the coil. Here, assuming the diameter andsurface area of coil 108 and stent 100 are nearly equal, the current inthe stent is nearly equal to the current in the coil at t=0.

[0067] According to the laws of Biot Savard, two parallel wires oflength L carrying a current I₁ and I₂ and separated by a distance rexperience a force F=(I₁I₂L²10⁻⁷)/r². This force can be applied tocompress stent 100, also providing the stent with kinetic energy. Ascalculated above, the kinetic energy (½ m²) required is 0.094 Joule,which means that the initial velocity, v, of stent 100 is 166 m/s. (Themass of the stent is 6.8×10⁻⁶ kg.) Assuming a design in which a singleturn coil is used, i.e., a flat wire as wide as the length of stent 100,the current I₁ through the coil is nearly equal to the current I₂through the stent. The net radial force acting on the stent isF=10⁻⁷I²(π²R₁R₂)/R₂−R1)²−3 N. Three Newtons is the elastic force of thestent. The speed of the stent is equal to the integral of the forcedivided by the mass.

[0068] Using numerical solution, and assuming time steps of 10-6seconds, sending 1500 Amperes through the coil can result in a maximumspeed of 152 m/s. Commercial high current pulse generators can deliverup to 2400 Amperes. The current can also be reduced to crimp in steps.For example, a current of 200 Ampere can reduce the radius from 2 mm to1.54 mm.

[0069] At the same time, part of the induced current in stent 100 can bedissipated into heat by Ohmic losses. The heat dissipation duringcrimping is P=I²Rdt, where R=pL/A. Here, p=1.676×10⁻⁵ W cm (for steel),L=0.1257 cm, A=0.008 Cm² (thickness of stent=0.01 mm, length=0.8 cm), soR=2.62×10⁻⁴ Ohm. Heat dissipation is 590 Watts. The time period to reach150 m/s is 5×10⁻⁶ seconds. Therefore, the amount of heat dissipatedduring this time period is 0.003 Joule, which is relatively small,suggesting that most of the energy can be used to compress the stent.

[0070] The above calculations do not, e.g., account for decay of theinduced current, but the example provides one illustrative guideline forsecuring an endoprosthesis to a catheter.

[0071] Other embodiments are within the claims.

What is claimed is:
 1. A method of making a medical device, the methodcomprising: electromagnetically compressing a prosthesis to a catheter.2. The method of claim 1, comprising electromagnetically compressing theprosthesis on an inflatable balloon on the catheter.
 3. The method ofclaim 2, further comprising heating the balloon.
 4. The method of claim1, further comprising positioning a restraining device over thecompressed prosthesis.
 5. The method of claim 1, wherein the prosthesisis a balloon-expandable stent.
 6. The method of claim 1, wherein theprosthesis is a balloon-expandable stentgraft.
 7. The method of claim 1,wherein the prosthesis is a self-expandable stent.
 8. The method ofclaim 1, wherein the prosthesis is a self-expandable stent-graft.
 9. Themethod of claim 1, wherein the prosthesis includes a therapeutic agent.10. The method of claim 1, comprising electromagnetically compressingdifferent portions of the prosthesis with different forces.
 11. Themethod of claim 1, further comprising supporting the prosthesis on thecatheter.
 12. The method of claim 1, further comprising axiallytranslating the prosthesis.
 13. The method of claim 1, furthercomprising positioning a mandrel in the catheter.
 14. A medical devicemade according to the method of claim
 1. 15. A method of making amedical device, the method comprising: positioning a prosthesisincluding a therapeutic agent on an expandable portion of a catheter;and electromagnetically compressing the prosthesis.
 16. The method ofclaim 15, further comprising positioning a mandrel in the catheter. 17.The method of claim 15, wherein the prosthesis is selected from a groupconsisting of a balloon-expandable stent and a balloon-expandablestent-graft.
 18. The method of claim 15, further comprising heating theexpandable portion.
 19. The method of claim 15, comprisingelectromagnetically compressing different portions of the prosthesiswith different forces.
 20. A medical device made according to the methodof claim
 15. 21. A method of making a medical device, the methodcomprising: compressing a prosthesis to a catheter without contacting anouter surface of the prosthesis.
 22. The method of claim 21, comprisingcompressing the prosthesis on an inflatable balloon on the catheter. 23.The method of claim 22, further comprising heating the balloon.
 24. Themethod of claim 21, further comprising positioning a restraining deviceover the compressed prosthesis.
 25. The method of claim 21, wherein theprosthesis is selected from a group consisting of a balloon-expandablestent, a balloon-expandable stent-graft, a self-expandable stent, and aself-expandable stent-graft.
 26. The method of claim 21, wherein theprosthesis includes a therapeutic agent.
 27. The method of claim 21,comprising compressing different portions of the prosthesis withdifferent forces.
 28. The method of claim 21, further comprisingsupporting the prosthesis on the catheter.
 29. The method of claim 21,further comprising axially translating the prosthesis.
 30. The method ofclaim 21, further comprising positioning a mandrel in the catheter. 31.A medical device made according to the method of claim
 21. 32. A methodof making a medical device, the method comprising: securing a medicalballoon to a catheter; and subsequently securing a marker band under theballoon to the catheter.
 33. The method of claim 32, wherein the band issecured to the catheter without mechanically contacting the band. 34.The method of claim 32, wherein the band is secured to the catheterelectromagnetically.
 35. The method of claim 32, wherein the band isvisible by magnetic resonance imaging.
 36. The method of claim 35,wherein the band is formed of a material having dysprosium orgadolinium.
 37. The method of claim 32, wherein the band is formed of amaterial selected from the group consisting of gold, platinum, tungsten,and tantalum.
 38. A method of making a medical device, the methodcomprising: securing a marker band to a support without contacting anouter surface of the band.
 39. The method of claim 38, wherein thesupport is selected from the group consisting of a catheter, a medicalballoon, a guidewire, and a stent.
 40. A method, comprising: providingan endoprosthesis including a metal body and a polymer layer; andreducing the size of the endoprosthesis without contacting theendoprosthesis.
 41. The method of claim 40, wherein the polymer layerincludes a drug.
 42. The method of claim 40, wherein the polymer layeris on an outside surface of the metal body.
 43. The method of claim 42,wherein the endoprosthesis is reduced in size without contacting thepolymer layer.
 44. The method of claim 40, wherein the endoprosthesis isa stent.
 45. The method of claim 40, further comprising securing theendoprosthesis to a support.
 46. The method of claim 40, wherein thesupport is selected from the group consisting of a catheter and amedical balloon.